In a conventional servo motor system, the driver circuitry and the indexer circuitry are in the form of printed circuit boards (PCB) with integrated circuits and other discrete components soldered to copper traces that are photochemically etched on the board. In addition, the driver and indexer circuitry in a high resolution servo motor positioning system is typically complex in comparison to the circuitry found in the simple speed controlled servo motor systems in which a tachometer is typically used to control the speed of the motor. In a PCB configuration, the driver in a servo motor system is typically large and bulky, thereby requiring a separate enclosure that is unattached to the motor and connected to the motor by a lengthy cable. An encoder, resolver or some other position/velocity sensor system is provided and is connected to the driver. The output supplied by the position/velocity sensor system allows the driver to control the position and velocity of the motor. Connected to the driver is an indexer which tells the driver how far, and at what speed, to advance the motor. In addition, a separate power supply is connected to the driver and indexer.
There are inherent disadvantages in a conventional servo motor system, however.
1. Size
Conventional driver circuits in a servo motor system are comparatively large. A high power, high speed, or fine resolution servo motor system is bulky and can not be reduced to a small enough size to be placed in a restrictive area, or attached directly to a motor. The addition of an indexer or external power supply significantly adds to the size of the system.
2. Weight
When populated with components and their heat sinks, the weight of the driver and indexer circuits in an appropriate enclosure can significantly reduce the benefits of using a conventional servo motor system in weight sensitive industries such as the aerospace and aviation industries.
3. Heat Dissipation
The substantial heat dissipation requirements of a conventional servo motor driver circuit prevent the driver from being mounted in restrictive areas or enclosures without ventilation. Conventional driver circuitry is complex and has many individual components populating a driver circuit, each having its own heat dissipation requirements. Because of the need to dissipate the heat of the individual components through the use of bulky heat sinks, a conventional driver circuit must be large by nature. Even with substantual heat sinking, a conventional driver can only be mounted in low temperature environments because of the relatively ineffective nature of a typical heat sinking system.
4. EMI/RF Emissions
Because of the large area on a PCB in which traces and components are exposed, a servo amplifier that utilizies high frequency, current switching techniques will emit EMI/RF that can damage or interfere with EMI/RF sensitive devices located near the PCB. Many industries, such as the health care and semiconductor manufacturing industries, cannot use conventional servo motor systems because of the EMI/RF noise they emit. In addition, as in the case of a driver located near an electrical motor, EMI/RF produced by the motor may damage or interfere with any exposed EMI/RF sensitive devises located on the driver, or indexer.
5. High-Frequency Performance
Because of variances in trace width, length and exposure, a PCB circuit is limited in its ability to handle high speed transmission of signals. As in the case when a motor makes positioning moves that are high in resolution, a driver with limited high speed signal transmission capabilities would restrict a motor's speed, as well as limit the resolution a motor could possibly achieve. (Example: 4,000 steps per revolution verses 50,000 steps per revolution.) An additional problem occurs when the motor is not in very close proximity with the high resolution driver. The greater the distance that the signal must travel to the motor, the lower the possible speed and resolution a motor can achieve.
6. Circuit Performance
The performance of a PCB driver circuit is limited by the tolerances of the discrete resistors, capacitors, inductors, and conductors used in the design, as well as the trace width, length, thickness and exposure variances on the PCB. The inability to fine tune the resistors, capacitors, inductors, conductors, and traces results in a circuit that is less efficient, less predictable, less reliable, less stable and with broad absolute specifications.
7. Fixed Operating Parameters
In conventional servo motor systems, the distance between the motor, driver and indexer makes it difficult to monitor small, but significant changes in the real time operating parameters, such as temperature and current, occuring in the motor system. The inability to monitor these parameters has resulted in the inability to safely optimize a motor's operating efficiency as determined by certain changes in its operating environment.
8. Assembly
Assembling a conventional servo motor system in a product is time consuming, requiring manual placement and wiring. Errors can frequently occur when placement and wiring is done manually. These errors will often damage the system, and in some cases can cause personal injury.
9. Environment
Conventional driver and indexer circuits are not hermetically sealed. Exposure to humid environments will damage the circuits. In flammable environments, a short in the circuit could cause an explosion.
10. Safety
Damage done to a PCB drive circuit as a result of a hostle environment, or severing the external connecting wires, or inadequate heat dissipation can result in life threatening safety hazards, such as electrocution.
11. Mechanical Stability
In a conventional servo motor system, the individual components on a driver circuit are not permanently potted in place, and may become loose as a result of vibration.
A conventional servo motor system is a closed loop system that requires an AC or DC motor, a driver, a position/velocity sensor system, such as an encoder or resolver (which monitors the position, speed, and direction of a move), or tachometer (which only monitors the speed of a move), and an indexer. The motor is actuated by the driver circuitry which contains a current control means, a preamplifier means, and a translating means. The indexer which is connected to the driver sends information to the translator. The information signals the desired characteristics of a specific move, such as the distance, speed, or direction of the move. In conventional systems, the information is sent to the driver in the form of a single move command. In some sophisticated systems, the information is sent to the driver in the form of a number of individual move pulses, whereby each pulse moves the motor a specific distance until the desired position is achieved. The translator then regulates the current control means by way of the preamplifier means, thereby controlling the flow of current into the windings of the motor. The position/velocity sensor system provides feedback to the driver which uses the feedback to control the motor in a manner that achieves the desired characteristics of the specific move. In some sophisticated configurations, the indexer also contains circuitry for communicating with outside controls, thereby allowing a computer to send variable move commands to the indexer.
In closed loop positioning applications, the position/velocity sensor is typically in the form of an optical encoder or resolver that is connected to the motor. The encoder or resolver provides feedback to the encoder or resolver interface circuitry, which translates the information to a means understandable by the translator means. The information can then be transmitted to the translator enabling the translator to control the flow of current into the windings of the motor in order to achieve the desired position, speed, and direction of a move.
There appears to be a trend to miniaturize. For example, in a prior art servo motor system manufactured by Copley Controls Corporation of Newton, Mass., a current control means is available in a standard dip package. Although the current control means is substantially larger than a standard intergrated circuit (IC) semiconductor, the current control means can be mounted along with other driver, indexer and encoder or resolver circuitry on a single printed circuit board. However, the Copley current control means maintains the limitations of a conventional servo motor system. For example, the size of the Copley unit remains relatively large and bulky, and must remain separated from the motor itself. The unit still requires relatively lengthy external wires between the driver means and motor. Faulty connections and wiring errors can still occur, possibly damaging the components in the circuit or causing electrocution. The unit still requires an external power supply, which must be manually wired to the unit. In addition, the unit and their associated wiring radiates electromagnetic interference that should be reduced, if possible. Finally, the unit and power supply still requires a relatively large amount of space, as well as the adaptability of the space to handle a significant amount of weight and heat dissipation.
Galil Motion Control, Incorporated of Mountain View, Calif., has developed an integrated circuit (IC) semiconductor that replaces portions of the translator and servo preamplifier in conventional servo motor systems. The IC, however, still requires additional driver circuitry, power supply, and position/velocity sensor, thereby maintaining many of the problems associated with a conventional servo motor system.
Some companies, such as Vernitron Control Components of San Diego, Calif., manufactures synchros, or transducers, which have accurate positioning capabilities. Synchros are primarily used in military applications, and are actually synchronous motor systems (or generators) which have been adapted to serve as variable transformers in the measurement of angular position. The actuator, actuating circuitry, and resolver are combined in a single unit. The actuating circuitry, which is in printed circuit board form, is potted in place. The unit, however, has many limiting features. For example, the unit is a low power unit that requires a large external power supply, which substantially adds to the total size and weight of the system. The unit is not designed to for use in positioning applications that require moderate to high levels of holding torque, such as 100 oz. in. to 1000 oz. in., that are typical in the robotic and automation industries. ( A synchro, or transducer, typically has a maximum no load holding torque of less than 20 oz. in.) The speed of the unit is generally less than 400 RPM which is substantually slow when compared with the speed of a conventional servo motor system. In addition, the positioning accuracy of the unit is low when compared with the positioning accuracy of a conventional servo motor system. These are only a few of the limting aspects of a synchro, or a transducer.
It is an object of the present invention to provide a novel motor drive system that alleviates many of the problems concomitant with the motor drive systems of the prior art.
A further object of the present invention is to provide a motor system that monitors the operating parameters of the motor system.
Another object of the present invention is to provide a servo motor system that is significantly reduced in size and weight from the servo motor systems of the prior art.
A further object of the present invention is to provide a servo motor system that has less heat generated than the heat generation of prior art servo motor systems.
A still further object of the present invention is to provide a servo motor system that enables significantly reduced EMI/RF emissions.
Another object of the present invention is to provide a servo motor system in which assembly and connecting operations of the components are relatively easy to perform.
A further object of the present invention is to provide a servo motor system that is compact in size and is effectively mounted within a small space.
Another object of the present invention is to provide a servo motor system that is relatively simple in construction and easy to manufacture.
A still further object of the present invention is to provide a servo motor system that has significant mechanical stability.
Another object of the present invention is to provide a servo motor system that dissipates heat more efficiently than the prior art.
A further object of the present invention is to provide a drive package that can be easily adapted to servo motors of various shapes and sizes.
A still further object of the present invention is to provide a motor, drive circuit, control circuit, and position/velocity sensor in a unitary device.
Other objects and advantages of the present invention will become apparent as the description proceeds.